One of the most controversial issues of concern to public health and water resources personnel is the relation between drinking water and cancer. This problem was highlighted when Robert Harris of the Environmental Defense Fund reported that higher rates of total cancer, cancer of the urinary system and of the gastrointestinal tract were observed among individuals drinking treated surface water from the Mississippi River in Louisiana, New Orleans area, compared to others, drinking ground water in the same state. Significant, however, has been the rapid shift from the basic issue, i.e., is the difference observed in cancer rates among the users of different water supplies limited to Louisiana or is it initial evidence of a more general phenomena? Apparently the Louisiana observation has been accepted as a causal association, since focus has shifted to a search for the particular water contaminant(s) responsible for the observed differences, specifically, to the health risk from waterborne chloro-organic compounds, some of which were shown to be a product of chlorine-disinfection process. Although evaluation of the health risk of water chlorination has merit, the narrowing of the entire research focus on the matter of chlorination has been hasty. If the observations in Louisiana hold true in other areas, it increases the likelihood of the existence of a water related causal factor which may well be other than chlorination. The fact is, however, that the evidence of the link between cancer and source of drinking water is insufficient for acceptance. A promising and appropriate line of investigation would be to critically and thoroughly examine this basic issue by analyzing cancer mortality rates among the users of both categories of water supplies. The objective of this proposal is to examine cancer mortality and patterns of water utilization in Texas, and to assess the cancer risk in populations using surface and ground sources of drinking water within and among Texas' 254 counties, with geographic references to specific river basins and major ground water aquifers from which the supply is derived.